1. Field of the Invention
The present invention relates to a color image forming apparatus, such as an electrophotographic type or electrostatic recording type copying machine or printer.
2. Description of the Related Art
In a rotary type image forming apparatus, a development rotary rotates for every rotation of an intermediate transfer member. Furthermore, visible images of yellow, magenta, cyan, and black are serially formed on a photosensitive drum by using a yellow development unit, a magenta development unit, a cyan development unit, and a black development unit in this order.
In addition, the formed visible images are transferred during four rotations of the intermediate transfer member. In this manner, color images are formed on the intermediate transfer member. More specifically, color images can be formed by transferring each toner at the same position of the intermediate transfer member.
Japanese Patent Application Laid-Open No. 2000-66475 discusses a method capable of forming each color visible image at the same position of an intermediate transfer member by detecting and using a reference mark provided on a periphery of the intermediate transfer member with an optical sensor, which outputs an image forming reference signal (hereinafter simply referred to as a “/TOP signal”) as a signal indicating an image forming start position on the intermediate transfer member based on timing to detect the reference mark.
In the following description, /TOP signals of four colors of yellow, magenta, cyan, and black, based on the timing to detect a reference mark with the optical sensor, are defined as a “/TOP signal Y”, a “/TOP signal M”, a “/TOP signal C”, and a “/TOP signal k”, respectively.
Meanwhile, because the size of an image forming apparatus has recently been reduced, the peripheral length of an intermediate transfer member of a recent image forming apparatus has been short. In this case, the peripheral length of the intermediate transfer member is only slightly longer than the longitudinal size of a maximum-size sheet on which an image can be formed by such a recent image forming apparatus.
Under these circumstances, if the difference between the peripheral length of an intermediate transfer member and the paper size is small, switching of a current development unit to a subsequent development unit may not be completed before detecting a subsequent reference mark. In this case, a /TOP signal of a subsequent color is output after an idle running of the intermediate transfer member for one rotation.
FIG. 12 is a timing chart illustrating timing of control for idling an intermediate transfer member by one rotation. Referring to FIG. 12, a print reservation command for two pages is transmitted from a controller unit 201 to an engine control unit 202. After receiving a print start command 1 (timing 305) corresponding to a print reservation command for one page (timing 303), the engine control unit 202 starts preprocessing rotation sequence.
After the preprocessing rotation sequence is completed (timing 320), if a reference mark is detected (timing 330), a /TOP signal Y1 is output (timing 340) and an operation for printing a first page starts.
The controller unit 201, in synchronization with the output of the /TOP signal Y1 (timing 340), transmits image data of yellow to the engine control unit 202 (timings 350 and 351). Furthermore, the engine control unit 202 executes control for contacting a development unit and a photosensitive drum with each other according to a paper size designated in the print reservation command 1 (the timing 303). After that, the engine control unit 202 causes the development unit to separate from the photosensitive drum and switches the development unit from a development unit Y to a development unit M (timing 321).
When a reference mark is detected (timing 331), the development unit is currently being switched from the development unit Y to the development unit M. Accordingly, a /TOP signal (M1) cannot be transmitted at timing 341.
After the switching of the development unit is completed at timing 322, in synchronization with timing to detect a reference mark after one rotation of the intermediate transfer member (timing 332), a /TOP signal M1 is output (timing 342). After that, the development unit is switched (timings 323 and 325) for /TOP signals C and K1 (respectively output at timings 344 and 346) in a similar manner.
At timing to detect a reference mark during switching of the development unit (timings 333 and 335), the /TOP signals C1 and K1 (output at timings 343 and 345 respectively) are not transmitted.
After completing the switching of the development unit (timings 324 and 326), in synchronization with the detection of the reference mark by one rotation of the intermediate transfer member (timings 334 and 336), /TOP signals C1 and K1 are output (timings 344 and 346).
The controller unit 201 transmits M, C, and K image data of the first page to the engine control unit 202 (at timings 360 and 361, 370 and 371, and 380 and 381, respectively). After having completely transmitted all the image data of four colors, a print start command 2 is output (timing 306), which corresponds to a print reservation command 2 for a second page (output at timing 304).
After receiving the print start command 2 (timing 306) and after image forming K1 of the first page is completed (timing 381), the engine control unit 202 switches the development unit from the development unit K to the development unit Y (timing 327). At timing at which a reference mark is detected during switching of the development unit (timing 337), a /TOP signal Y2 is not transmitted (timing 347).
After having completed the switching of the development unit (timing 328), in synchronization with timing at which a reference mark is detected by one rotation of the intermediate transfer member (timing 338), a /TOP signal Y2 is output (timing 348). M, C, and K images are formed on subsequent pages in the same manner as forming the same on the first page.
If neither a print reservation command nor a print start command for a third page have not been received, the engine control unit 202 starts post processing of a printing operation (hereinafter simply referred to as “post processing rotation sequence”) and ends the printing operation.
As described above, if timing to detect a reference mark comes during switching of a development unit, it becomes necessary to idle the intermediate transfer member by one rotation to delay the timing of transmitting a /TOP signal. Japanese Patent Application Laid-Open No. 2006-145595 discusses a method for preventing unnecessary idling of an intermediate transfer member by reducing the traveling speed of an intermediate transfer member to a speed lower than a usual traveling speed.
As described above, in the above-described conventional method, idling of an intermediate transfer member is prevented by reducing the speed thereof. However, in a small size image forming apparatus, it is difficult to change the traveling speed of the intermediate transfer member. Accordingly, if the development unit cannot be switched before detecting a reference mark, the idling of the intermediate transfer member becomes always necessary.
If the above-described control is executed, the throughput becomes low. Accordingly, it is desired by the market to improve the throughput.